1. Field of the Invention
The present invention relates to screen assemblies for rear projection systems and more particularly to minimizing secondary ghost images and providing enhanced contrast.
2. Description of the Related Art
Recently low profile (thin) microdisplay-based rear projection monitors and digital televisions have been developed. A Fresnel lens is typically used behind the viewing screen for rear projection systems. The purpose of the Fresnel lens is to “collimate” the light rays incident from the projection lens, in order to minimize luminance nonuniformities near the edges of the display. The Fresnel lens is comprised of a series of concentric facets which become increasingly steep towards the edges of the display, providing the increased refraction necessary for “collimation”. Additionally, many rear projection applications require minimal depth, for space and weight savings. As the distance from the projection lens to the screen decreases, the angles of incidence of the projected rays approaching the edges of the screen correspondingly increase for a given screen size, increasing the required “power” of the Fresnel lens (reducing its f/number) in order to provide the needed light steering to achieve collimation of the off-axis rays. This increases the steepness of the facets and can cause undesirable artifacts, namely secondary/ghost images, which are present along with the primary projected image. As will be disclosed below, the present invention discloses ways to integrate a viewing screen with a short focal length Fresnel lens in a durable assembly, to suppress these ghost images.
U.S. Pat. No. 6,483,612 (US20010001582), U.S. Pat. No. 6,768,566 (US 20020159109), and each entitled “Projection Screen Apparatus Including Holographic Optical Element” and each issued to D. S. Walker, disclose a screen apparatus that includes a holographic optical element and a diffuser. The holographic optical element may be constructed using standard techniques known in the field of holography. The holographic optical element may be used to replace a typical Fresnel lens used in projection screen apparatuses. In operation, the holographic optical element receives image light from an image engine or projector and redirects the image light to the diffuser for scattering. The holographic optical element can be designed to substantially collimate, converge, or diverge the image light. The combination of the holographic optical element and the diffuser provides improved illumination uniformity that can be perceived by a viewer as the viewer moves in directions transverse to the screen apparatus. The screen apparatus may be designed to provide improved illumination uniformity to optimized or optimal locations in a viewing region. The screen apparatus may be advantageously employed in display apparatuses. Some of the disadvantages of this approach are (i) holograms are typically prone to speckle which is distracting to the viewer (ii) the efficiency of the hologram is compromised somewhat at extreme angles (iii) for short throw (low f/#) systems this approach would be prone to ghosting as well and (iv) the engineering/setup costs are higher versus conventional approaches using the Fresnel lens.
U.S. Pat. No. 6,710,941 (20020154401) and U.S. Pat. No. 6,407,859, each entitled “Fresnel Lens For Projection Screens” and each issued to D. W. Hennen et al, disclose a screen, such as is used in back-lit projection screens, having a Fresnel lens laminated to another layer for support. The screen includes a Fresnel lens having an output surface, and a dispersing screen supportingly attached on a first side to the output surface of the Fresnel lens. The main disadvantage of this approach is that a portion of the “active” facets responsible for the actual light refraction/bending is not available as it is used up in supporting the Fresnel lens. Furthermore, while this approach is intending to get the Fresnel lens as close to the screen as possible to minimize ghosting, the present invention, as will be disclosed below, achieves the same result but is not subject to these limitations. More specifically, the screen can be separated from the Fresnel lens and still be free of ghosting since the ghost reflections have been “locally” suppressed in the modified Fresnel subassembly.
Many applications require the screen to be viewed in high ambient lighting, where incident illumination reflected from the screen can lower contrast and make the display hard to read. Often, high contrast beaded screens are utilized with these high ambient viewing situations because they have low reflectivity and relatively high transmittance. Such screens are described in U.S. Pat. No. 2,378,252, entitled “Projection Screen”, and issued to H. C. Staehle, et al., and in U.S. Pat. No. 55,563,738, entitled “Light Transmitting and Dispersing Filter Having Low Reflectance”, issued to D. W. Vance. In systems utilizing such screens, a Fresnel lens collimates the projection lens output. This light is then refracted by the beads and exits through small apertures on the viewer side. The incident ambient is largely absorbed by the black resin, resulting in excellent high ambient contrast. Regardless of bead size, the screen structure is visible and can be objectionable to the viewer (grain). Additionally, imperfect beads create bright spots which vary as the viewer's head position is changed (scintillation). As will be disclosed below, the current invention minimizes the visual artifacts and provides superior high ambient contrast.
There have been other attempts to provide high contrast rear projection screens. For example, U.S. Pat. No. 6,449,089 B1 entitled “Rear Projection Screen with Enhanced Contrast” discloses projection screen assemblies that include one or more birefringent retarding layers and one or more polarizing layers. The retarding and polarizing layers reduce the unwanted reflection of ambient and image light. Embodiments show utilization of a serial arrangement of a dispersing (i.e. diffusing) layer substrate, a quarter wave retarder, and a linear polarizer for contrast enhancement. However, the '089 screen assemblies do not include implementations to mitigate ghosts observed in certain rear projection systems. This is especially applicable to systems which are thin, demanding the use of a high power Fresnel lens which is prone to ghost generation.
Other attempts to provide high contrast screens include U.S. Pat. No. 6,519,087, entitled “Rear Projection Screen Incorporating Diffuser”, issued to R Moshrefzadeh. In this invention a diffuser is added to a beaded screen to mask the appearance of grain. The drawbacks to this approach are that system resolution is degraded by the diffuser, and screen reflectance is increased. Another approach is described in U.S. Pat. No. 6,636,355B2, entitled “Microstructured Rear Projection Screen” and issued to R. Moshrefzadeh et al. A drawback of this approach is that virtually any structure is visible when the screen is viewed from a short distance, as would be the case in vehicular applications, or desktop or medical applications, to name a few.